Democratic presidential candidate Hillary Clinton speaks during a town hall meeting in Las Vegas, Nev. Aug. 18, 2015. Mrs. Clinton on Tuesday came out against the Obama administration in voicing her opposition to Shell’s exploratory drilling for oil and gas in the Arctic.

On Tuesday, the former secretary of state voiced her opposition to Royal Dutch Shell’s quest for oil in the region, putting her at odds with the Obama administration, which approved the company’s final permits the day before. The statement is also Mrs. Clinton’s strongest position so far on an issue that some environmentalists consider critical in determining a presidential candidate’s commitment to addressing climate change.

“The Arctic is a unique treasure,” Clinton posted on her Twitter page Tuesday morning. “Given what we know, it’s not worth the risk of drilling.”

After being forced to shut down a similar project in 2012, Shell began exploratory drilling for oil and gas in the Arctic on July 30 on the condition that drilling be kept to the top of the seabed – thus avoiding the actual oil reserves – until the company had access to a “capping stack” that could shut down a well in case of a spill. The capping stack arrived last week, clearing the way for the administration’s final approval.

Arctic drilling is a key election issue for many environmentalists, who see oil and gas development in the region as a threat to the area’s ecosystems as well as a move that could deepen the United States’ reliance on fossil fuels.

Conservation groups have criticized President Obama for moving forward with the project while at the same time pledging sweeping action to mitigate climate change: Earlier this month, he revealed his signature policy to curb power-plant emissions, and last week announced a trip to Alaska to visit communities affected by climate change.

“The president cannot have it both ways,” Annie Leonard, executive director of Greenpeace USA, told The Guardian. “Announcing a tour of Alaska to highlight climate change days before giving Shell the final approval to drill in the Arctic ocean is deeply hypocritical.”

Which may be part of why Clinton has decided to stand against Arctic drilling, despite her vocal support for most of Obama's climate policies.

The Democratic frontrunner has already faced criticism for her reticence to take a position on the Keystone XL pipeline – a nearly 1,200-mile-long project to move oil from Canada to refineries in the US that has sparked heated debate between the oil and gas industry and conservationists, as The Christian Science Monitor has reported.

If Clinton ever reveals her position on the pipeline, she knows she will upset a large group of potential voters and donors. Businesses and organized labor want the pipeline bill to pass because of the jobs it will create. Environmental groups are staunchly opposed because of the damage it could do to the environment.

Taking a firm position against drilling in the Arctic – in addition to releasing a plan that outlines a nationwide shift to renewable energy sources – could strengthen Clinton’s image as a supporter of key environmental efforts while allowing her to steer clear of the Keystone XL issue.

“We applaud Secretary Clinton for standing up for what science, the will of the American people and common sense demand,” Michael Brune, executive director of the conservation group Sierra Club, told The Associated Press.

That hasn’t stopped Clinton’s Republican rivals from pushing back on her position on oil drilling. In response to her tweet Tuesday, former Florida Gov. Jeb Bush, who has said he is skeptical of climate change, countered: “Being more-anti energy than Obama is extreme. We should embrace energy revolution to lower prices and create US jobs.”

New Jersey Gov. Chris Christie, also a Republican candidate, chimed in: "Still waiting to hear your position on Keystone."

.@HillaryClinton Wrong. Being more-anti energy than Obama is extreme. We should embrace energy revolution to lower prices & create US jobs.